<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
		<style>
			
			.a{
				background-color: red;
				height: 200px;
				width: 200px;
			}
			.b{
				background-color: blue;
				width: 100px;
				height: 100px;
				display: none;
			}
		</style>
	</head>
	<body>
		<div class="a">
			A
			<div class="b">B</div>
		</div>
		
		
		<script src="js/jquery.3.6.3.js"></script>
		<script>
			
			$(".a").mouseover(function(){
				//鼠标移入事件
				$(".b").show();
			}).mouseout(function(){
				//鼠标移出事件
				$(".b").hide();
			})
		</script>
	</body>
</html>